Devices in which an acoustic beam and an optical beam interact are generally referred to as “acousto-optic devices”, or simply “AO devices”. In commercially available AO devices, the acousto-optic interaction occurs within the near field of the acoustic transducer. This is because the near field of the acoustic transducer is normally regarded as extending in a direction normal to the transducer surface for a distance of approximately one Rayleigh range (RL), which is given by the following equation:
                              R          L                =                                            (                              characteristic                ⁢                                                                  ⁢                length                            )                        2                    Λ                                    (        1        )            where the “characteristic length” is a length associated with some transverse dimension of the transducer, and Λ is the acoustic wavelength.
If the transducer electrode is rectangular (or has a pattern whose natural boundary is a rectangle), as is generally the case, there are in fact two characteristic lengths, conventionally taken as the transducer length (L) that by convention is in the optical propagation direction (OPD) of the beam in the AO crystal and the height (H) which is conventionally taken to be the direction transverse to the OPD referred to as the transverse direction. In this case the extent of the acoustic near field in the AO crystal is the minimum of the two Rayleigh ranges, found from equation (1) above using H and L as respective characteristic lengths.
As an example, consider a typical acousto-optic tunable filter (AOTF) having an electrode in the form of a rectangle of metal having L=20 mm and H=10 mm on a piezoelectric transducer. If the operating acoustic wavelength is 10 μms, the two Rayleigh ranges would be 40 meters (associated with L) and 10 meters (associated with H), respectively. In either case, since AO devices are known to be limited by available AO interaction crystal sizes to a few cms in size, device operation is always deep in the near field region. This situation is in stark contrast to other fields of acoustics, in sonar for example, where operation is nearly always in the far field region.
FIG. 1A represents a known AO device 100 comprising a conventional rectangular metallic top electrode 110 (generally referred to hereafter as “electrode” 110) over a piezoelectric (acoustic) transducer 115 having an AO interaction crystal 120 under the piezoelectric transducer 115 for receiving and propagating a light ray along the optical propagation direction (OPD) shown. Points A and B correspond to the left and right vertical side walls of the top electrode 110, that are parallel to the OPD. The acoustic field provided by the piezoelectric transducer 115 is assumed to be radiating in the direction perpendicular to the plane of the electrode 110 and that of FIG. 1A (i.e., radiating into the page). The OPD for the light ray received through the center of the aperture of this AO interaction crystal 120 propagates in the direction shown by the arrow (below the plane of the electrode 110).
FIG. 1B is simulated data that shows the effect of averaging the acoustic field along the OPD, row by row, for the known AO device 100 having the conventional rectangular electrode 110 represented in FIG. 1A. The relative average acoustic intensity is shown as a function of position in the transverse direction (transverse position) between points A and B. The center of the electrode 110 in the transverse (H) direction is shown as C in FIG. 1B.
The acoustic transducer was simulated as being 0.5 mm (L)×0.5 mm (H) in size (area), and the acoustic wavelength was set at 10 microns. The Rayleigh-Sommerfeld diffraction integral was used with no approximations (e.g., no assumption that the Fresnel approximation is valid) to calculate the acoustic field produced by the acoustic transducer, implemented as a numerical routine. The distance from the transducer 115 into the AO crystal 120 used in the simulation was approximately 0.5× H H, i.e. 0.25 mm.
It is noted that the electrode 110 defines the part of the acoustic transducer which produces acoustic waves, so for practical purposes it is often referred to as “the transducer”. However, for practical reasons in manufacturing, and as shown in FIG. 1A, the transducer 115 is usually larger in area as compared to the electrode 110.
It is noted that the integrated acoustic effect is not the same as the averaged effect. However, the averaged acoustic effect is used here for illustration as it is similar, and an AO device constructed according to the constraint of homogenizing the integrated acoustic effect will for practical purposes, be equivalent to one constructed by constraining the average acoustic field experienced by a photon as it passes along the OPD shown in FIG. 1A.
FIG. 1B shows significant variation in the length-averaged relative acoustic intensity along the transverse direction generated by known AO device 100 comprising a conventional rectangular electrode 110. For example, the peak-to-peak length-averaged variation in average acoustic intensity in the AO crystal 120 below a midway position between C of the electrode 110 and its edges (A, B) and transverse positions proximate thereto (defined herein a transverse position range of ±5 percent of H) can be seen to be at least about 20 percent of the value of the acoustic intensity at C (shown equal to 0.8).
The near acoustic field region generated by the transducer 115 acting as an acoustic radiation source thus possesses the characteristic of being very inhomogeneous as a function of transverse position, such as compared to its far field, a fact that often not considered in the design and operation of AO devices such AOTFs. This means that the acoustic intensities present in the acoustic near field region vary significantly from one transverse position to another. As a result, a ray propagating in the AO crystal 120 laterally from under the center (the center is shown as C in FIG. 1B) of the electrode 110 in the transverse direction may experience a highly non-uniform acoustic field in the AO crystal so that the acoustic field intensity may be up to 30% or more higher or lower as compared to the acoustic field intensity under locations at or near C.